The present invention relates in general to semiconductor power device technology, and in particular to structures and methods for forming a monolithically integrated trench gate field effect transistor (FET) and Schottky diode.
In today's electronic devices it is common to find the use of multiple power supply ranges. For example, in some applications, central processing units are designed to operate with a different supply voltage at a particular time depending on the computing load. Consequently, dc/dc converters have proliferated in electronics to satisfy the wide ranging power supply needs of the circuitry. Common dc/dc converters utilize high efficiency switches typically implemented by power MOSFETs. The power switch is controlled to deliver regulated quanta of energy to the load using, for example, a pulse width modulated (PWM) methodology.
FIG. 1 shows a circuit schematic for a conventional dc/dc converter. A PWM controller 100 drives the gate terminals of a pair of power MOSFETs Q1 and Q2 to regulate the delivery of charge to the load. MOSFET switch Q2 is used in the circuit as a synchronous rectifier. In order to avoid shoot-through current, both switches must be off simultaneously before one of them is turned on. During this “dead time,” the internal diode of each MOSFET switch, commonly referred to as body diode, can conduct current. Unfortunately the body diode has relatively high forward voltage and energy is wasted. To improve the conversion efficiency of the circuit, a Schottky diode 102 is often externally added in parallel with the MOSFET (Q2) body diode. Because a Schottky diode has lower forward voltage than the body diode, Schottky diode 102 effectively replaces the MOSFET body diode. The lower forward voltage of the Schottky diode results in improved power consumption.
For many years, the Schottky diode was implemented external to the MOSFET switch package. More recently, some manufacturers have introduced products in which discrete Schottky diodes are co-packaged with discrete power MOSFET devices. There have also been monolithic implementations of power MOSFETs with Schottky diode. An example of a conventional monolithically integrated trench MOSFET and Schottky diode is shown in FIG. 2. A Schottky diode 210 is formed between two trenches 200-3 and 200-4 surrounded by trench MOSFET cells on either side. N-type substrate 202 forms the cathode terminal of Schottky diode 210 as well as the drain terminal of the trench MOSFET. Conductive layer 218 provides the diode anode terminal and also serves as the source interconnect layer for MOSFET cells. The gate electrode in trenches 200-1, 200-2, 200-3, 200-4 and 200-5 are connected together in a third dimension and are therefore similarly driven. The trench MOSFET cells further include body regions 208 with source region 212 and heavy body regions 214 therein.
The Schottky diodes in FIG. 2 are interspersed between trench MOSFET cells. As a result, the Schottky diodes consume a significant portion of the active area, resulting in lower current ratings or a large die size. There is therefore a need for a monolithically and densely integrated Schottky diode and trench gate FET with superior performance characteristics.